The first cellulose crystalline particles were obtained through acid hydrolysis by Rånby et al. [1]. Later it was found that aqueous suspension of cellulose nanocrystalline particles can form a stable cholesteric (chiral nematic) liquid crystalline phase [2, 3]. The NCC particles are rod-like and of nanometer dimensions. In a dilute suspension, these nanocrystalline particles are randomly oriented. When the concentration of suspension is increased, it was believed that the cholesteric (chiral nematic) nanocrystals were formed and nanocrystals were helicoidally arranged, as shown in FIG. 1 [4]. Cholesteric liquid crystals display extremely high optical rotary power and reflect left hand of circularly polarized light. The reflected circularly polarized light wavelength λ=nP, where n is the mean refractive index of the chiral nematic phase, and P is the pitch of the chiral nematic structure. The wavelength of reflected light changes with the viewing angle, and iridescence is observed.
These cellulose nanocrystalline rods have unique physical properties such as high aspect ratio (10×200 nm), large surface area, and high tensile strength [5]. The shape and the nanometer-sized width of NCC rods allow a relatively flat film to be formed from the suspension by casting the suspension on an appropriate surface.
When water is evaporated the chiral nematic structure is preserved. Revol et al. [4] created a solidified liquid crystal film having advantageous optical characteristics. They adjusted the reflected visible light by adding different quantities of electrolyte such as NaCl or KCl. The formed solid films were anticipated to be supported on or embedded in a substrate. For example, small discs of the film may be embedded in security paper based on the optical properties. In their work, they stated that the cellulose nanocrystals were ideally suitable for an optical authenticating device. The film made as described in this patent was very brittle, without much flexibility. Beck et al. [6] discovered a method to control the iridescence colour of solid nanocrystalline cellulose (NCC) films by ultrasound or high-pressure shear (mechanical) energy input to the NCC suspension prior to film formation [6]. As the energy input to the NCC suspension increases, the colour of the resulting film shifts from the ultraviolet region towards the infrared region of the electromagnetic spectrum. This wavelength shift lies in the opposite direction to that caused by the addition of electrolytes to NCC suspensions prior to film formation. No additives are required to achieve the changes in colour. Colour changes can also be created by mixing two suspensions of differing sonication levels.
Beck et al. [6] also found that the iridescent colour of solid NCC films can be changed by controlling the pH and ionic strength of the NCC suspension. When the acid-form NCC (H-NCC) films are placed in sodium hydroxide solutions, their colour shifts towards longer wavelengths. This colour shift is partially reversed by re-placing the film in water. Although sodium-form NCC (Na-NCC) films can readily disperse in water, Na-NCC films do not disperse when placed in hydrochloric acid and sodium chloride, as well as sodium hydroxide solutions, of sufficient ionic strength; their iridescence also shifts toward longer wavelengths.
The above work focused exclusively on the manipulation and control of the optical properties of the NCC solid films. However, the solid films made with 100% NCC, as made and described in previous literature or patents, are very brittle and present handling difficulties, thereby reducing their suitability in many commercial applications. Prior to the present invention, there has been no method to produce flexible NCC film or enhance its flexibility.